The present invention relates to an apparatus as defined in the preamble of claim 1, and a method as defined in the preamble of claim 9, i.e. an apparatus and method for detecting a load decrease when driving piezoelectric elements.
Piezoelectric elements can be used as actuators because, as is known, they possess the property of contracting or expanding as a function of a voltage applied thereto or occurring therein.
The practical implementation of actuators using piezoelectric elements proves to be advantageous in particular if the actuator in question must perform rapid and/or frequent movements.
The use of piezoelectric elements as actuators proves to be advantageous, inter alia, in fuel injection nozzles for internal combustion engines. Reference is made, for example, to EP 0 371 469 B1 and to EP 0 379 182 B1 regarding the usability of piezoelectric elements in fuel injection nozzles.
Piezoelectric elements are capacitative elements which, as already partially alluded to above, contract and expand in accordance with the particular charge state or the voltage occurring therein or applied thereto. In the example of a fuel injection nozzle, expansion and contraction of piezoelectric elements is used to control valves that manipulate the linear strokes of injection needles. The use of piezoelectric elements with double acting, double seat valves to control corresponding injection needles in a fuel injection system is shown in German Applications DE 197 42 073 A1 and DE 197 29 844 A1, which are incorporated herein in their entirety.
Fuel injection systems using piezoelectric elements, e.g. as actuators, are characterized by the fact that, to a first approximation, piezoelectric elements exhibit a proportional relationship between applied voltage and the linear expansion. In a fuel injection nozzle, for example, implemented as a double acting, double seat valve to control the linear stroke of a needle for fuel injection into a cylinder of an internal combustion engine, the amount of fuel injected into a corresponding cylinder is a function of the time the valve is open, and in the case of the use of a piezoelectric element as an actuator, the activation voltage applied to the piezoelectric element.
FIG. 1 is a schematic representation of a fuel injection system using a piezoelectric element 2010 as an actuator. Referring to FIG. 1, the piezoelectric element 2010 is electrically energized to expand and contract in response to a given activation voltage. The piezoelectric element 2010 is coupled to a piston 2015. In the expanded state, the piezoelectric element 2010 causes the piston 2015 to protrude into a hydraulic adapter 2020 which contains a hydraulic fluid, for example fuel. As a result of the piezoelectric element""s expansion, a double acting control valve 2025 is hydraulically pushed away from hydraulic adapter 2020 and the valve plug 2035 is extended away from a first closed position 2040. The combination of double acting control valve 2025 and hollow bore 2050 is often referred to as double acting, double seat valve for the reason that when piezoelectric element 2010 is in an unexcited state, the double acting control valve 2025 rests in its first closed position 2040. On the other hand, when the piezoelectric element 2010 is fully extended, it rests in its second closed position 2030. The later position of valve plug 2035 is schematically represented with ghost lines in FIG. 1.
The fuel injection system comprises an injection needle 2070 allowing for injection of fuel from a pressurized fuel supply line 2060 into the cylinder (not shown). When the piezoelectric element 2010 is unexcited or when it is fully extended, the double acting control valve 2025 rests respectively in its first closed position 2040 or in its second closed position 2030. In either case, the hydraulic rail pressure maintains injection needle 2070 at a closed position. Thus, the fuel mixture does not enter into the cylinder (not shown). Conversely, when the piezoelectric element 2010 is excited such that double acting control valve 2025 is in the so-called mid-position with respect to the hollow bore 2050, then there is a pressure drop in the pressurized fuel supply line 2060. This pressure drop results in a pressure differential in the pressurized fuel supply line 2060 between the top and the bottom of the injection needle 2070 SO that the injection needle 2070 is lifted allowing for fuel injection into the cylinder (not shown).
It is important to determine and apply an activation voltage with sufficient precision such that, for example, a corresponding valve plug is accurately positioned at the appropriate time in the fuel injection cycle. Thus it is important to be able to detect various problems in the electrical circuit driving the piezoelectric elements. One such problem is a short circuit to the battery voltage within, or at the terminals of, one or more of the piezoelectric elements.
It is therefore an object of the present invention to develop the apparatus as defined in the preamble of claim 1 and the method as defined in the preamble of claim 9 to reliably detect a drop in the electrical load of one or more of the piezoelectric elements.
This object is achieved, according to the present invention, by way of the features claimed in the characterizing portion of claim 1 (apparatus) and in the characterizing portion of claim 9 (method).
These provide for:
an error signal is generated when a target voltage is reached more quickly than a predetermined minimum time interval; and for
applying a circuit to a piezoelectric element to charge or discharge it to a target voltage, then monitoring how quickly the target voltage is reached, and finally generating an error signal when the target voltage is reached more quickly than a predetermined minimum time interval.
Because the electrical load of a piezoelectric element is capacitive in nature, the voltage across the load cannot change instantaneously. Rather, the voltage across the capacitive load will increase as a function of the current applied to the piezoelectric element over time. This process of increasing the voltage across the piezoelectric element is referred to as xe2x80x9ccharging.xe2x80x9d Similarly, the capacitive load must xe2x80x9cdischarge,xe2x80x9d or decrease, its voltage as a function of time, as current is drawn from the piezoelectric element.
When the load of a piezoelectric element drops, the measured voltage reaches the applied xe2x80x9cdesiredxe2x80x9d voltage much more quickly than expected.
Thus, it has been determined, according to the present invention, that a load drop of a piezoelectric element can be detected by monitoring whether an applied xe2x80x9cdesiredxe2x80x9d voltage across the piezoelectric element is reached in less than a predetermined minimum amount of time. If such voltage is reached in less.than the minimum amount of time a signal is generated to indicate that a load drop has occurred for that piezoelectric element. Such signal can be used to initiate countermeasures that may be necessary or to facilitate troubleshooting, for example, in the repair shop, or for storing an error message in an electronic memory.
Advantageous developments of the present invention are evident from the dependent claims, the description below, and the figures.